Knowing the frame material will help.

Wood you can gently steam and then bend back to where it lays flat. Lightly steam is the key word. Anything too damp and it will warp more.

Plastic can be heated to pull flat, but depends a lot on what and how.

If it a canvas, try to see if pulling one corner doesnt make it flat. That means it needs a staple or 2 to pull it tight.